Liquid crystal displays (LCDs) are commonly used in cell phones, personal digital assistants, laptop computers, desktop monitors, and televisions. LCDs require a backlight. For full color LCDs, the backlight is a white light. The white point of the white light is typically designated by the LCD manufacturer and may be different for different applications. The white point is specified as a heated black body color temperature.
Common white light backlights use either a fluorescent bulb or a combination of red, green, and blue LEDs.
For medium and large backlights, such as for TVs and monitors, multiple LEDs of each color are used. Typically, a number of LEDs of one color are connected in series on a printed circuit board (PCB). Generally, in backlights, external current drivers are used, each driving one or more strings of red, green, or blue LEDs. The amount of current through an LED controls the brightness. Groups of RGB LEDs are typically mounted on a single PCB, and there may be multiple PCBs in a large LCD.
It is important to have color uniformity across the entire LCD screen. This has been typically achieved by “binning” each LED according to its characteristics and then combining binned red, green, and blue LEDs on a PCB such that only boards with closely matching white points are used in a single backlight. The process to create boards with uniform light characteristics is costly and time consuming. Furthermore, variations within a PCB and between PCBs are not fully suppressed.
As a further obstacle to color uniformity, the brightness of an LED changes over time and not all LEDs change the same amount. Thus, a backlight with good initial color uniformity will become progressively nonuniform over time. Another problem is that, when an LED in series fails and becomes an open circuit, all the LEDs in the series will stop receiving power. This creates additional nonuniformity.